1. Field of the Invention
This invention pertains generally to test apparatus, and more particularly, to such apparatus for repeatably generating electrical arcs for testing an electrical switching apparatus, such as an arc fault circuit interrupter (AFCI). The invention also relates to methods for testing AFCIs.
2. Background Information
Electrical switching apparatus, such as circuit interrupters (e.g. and without limitation, arc fault circuit interrupters (AFCIs), ground fault circuit interrupters (GFCIs), and current leakage detectors), are employed in a wide variety of electrical power circuits to protect people and/or equipment from various electrical faults, including short circuits, defective grounding and arcing between power conductors. To ensure the reliability and consistent behavior of such devices, various private and/or government agencies have put forward minimum safety specifications, as well as specifications for conducting tests of compliance with those safety specifications.
An unfortunate feature of wiring with current day plastic insulators used both within the walls of a structure and in open spaces (e.g. and without limitation, appliance cords and extension cords) is the formation, over time, of quantities of carbon molecules between electrical conductors or across gaps resulting from breaks in electrical conductors. The formation of these quantities of carbon molecules in such locations is often hastened by such wiring being subjected to repeated physical abuse, and this hastening most often occurs in extension cords or appliance cords lying across floor surfaces where they are repeatedly stepped on, rolled over with carts, pinched under or between pieces of furniture, etc. As such quantities of carbon molecules form between conductors or across the gaps of broken conductors, they can become intermittent conductors, themselves, causing electrical arcing to occur that has been known to start fires. One of the responses to this problem with these cords has been the creation of AFCIs that respond to the occurrences of such arcs by disconnecting power from wires in which arcing occurs in a manner not unlike that of a circuit breaker. In an effort to ensure the reliability of these AFCIs, Underwriters Laboratories, Inc. (UL), has promulgated standards for minimum characteristics of such AFCIs and for testing such AFCIs, including the UL1699 standard.
Section 58.1.3 of UL1699 specifies the details of an arc generator to be used in simulating arcs for the purpose of testing AFCIs. The specified design employs a rack and pinion with a hand crank to allow an operator to manually separate a carbon electrode and a copper electrode to create an arc. In normal use, an operator begins by bringing the carbon and copper electrodes into contact with each other before turning on the power, and then the operator very slowly separates the electrodes until the operator (wearing protective eyewear with darkened lenses) sees an electrical arc initiated between the electrodes. Unfortunately, creating arcs that last long enough to permit an AFCI to be properly tested with such a hand-operated apparatus has proven difficult. The typical distances by which these electrodes must be separated to initiate and maintain an arc are measurable in micrometers or fractions of a micrometer, and have a similarly minute range of error. Indeed, the distances are sufficiently minute and human hand-eye coordination is sufficiently sluggish, that it is often the case that by the time the operator is able to see, comprehend and respond to the initiation of an arc by ceasing to further separate the electrodes, the operator has already separated the electrodes by too great a distance, and the arc is extinguished only a moment after being initiated (what is commonly called a “blow-out”).
Even where an individual has developed considerable skill in using such a hand-operated apparatus to create sustained arcs with some regularity, there are still drawbacks. Given the previously mentioned minute tolerances for error, a hand-operated arc generator is not conducive to altering the distance between the electrodes by controllable amounts to provide either refinement of or control over the properties of an arc. Also, operator fatigue can quickly set in from having to do the delicate work of generating arcs a number of times to perform a range of different tests on even a single AFCI. There are also concerns that having an operator repeatedly watching the initiation of arcs, even with the protection of darkened lenses, may have damaging long term effects on eyesight. Furthermore, such a hand-operated apparatus requiring precision of hand-eye coordination to generate each arc is simply not conducive to carrying out tests of even limited production runs of AFCIs.